Method of making hollow castings



April 17, 1962 H. L. M CORMICK 3,029,485

METHOD OF MAKING HOLLOW CASTINGS Filed Jan. 14, 1959 2 Sheets-Sheet 1 ifINVENTOR.

April 17, 1962 H. L. MCCORMICK 3,029,485

METHOD OF MAKING HOLLOW CASTINGS Filed Jan. 14, 1959 2 Sheets-Sheet 2INVENTOR.

States 3 029,485 METHOD OF MAKJiNG HOLLOW CAT1NGS Hamilton L. McCormick,Carmel, Ind., assignor to General Motors Corporation, Detroit, Mich, acorporation of Delaware Filed Jan. 14, 1959, Ser. No. 786,863 4 Claims.(Cl. 22-160) This invention relates to a method for making hollowcastings and particularly to a method for making turbine blades andvanes, entrance guide vanes and the like which have fluid passagestherethrough to provide a cooling means for the blade during theoperation of the engine in connection with which it is used.

In turbojet engines a turbine operated by burning gases drives a blowerwhich furnishes air to the burner. Such turbines desirably operate atvery high temperatures and the capacity and efficiency of such enginesis limited by the ability of the turbine blade to Withstand the hightemperatures involved. To this end, blades are provided with passagestherethrough through which coolant fluids may be passed during operationof the engine. Casting blades of this type presents difiicult foundryproblems because the blades are relatively thin and have a markedlycurved configuration, and the coolant passages must be accuratelypositioned within the blade structure to efficiently perform theirintended function. The coring arrangement to be used in casting such ablade must be capable of withstanding physical handling and thermalshocks in the casting procedures, and must be capable of beingaccurately and firmly positioned within the investment mold cavity, andbe readily removable after casting without detrimental effect on themetal or blade airfoil geometry.

It is an object of this invention to provide a cast turbine blade or thelike of the hollow type having a plurality of passages extending fromthe tip thereof to a plenum chamber located within the platform portionof the blade and having a fluid entry passage leading to the plenumchamber through the root of the blade.

It is a further object of this invention to provide a method ofmanufacturing turbine blades, compressor blades and the like, having aplurality of passages therein at predetermined positions. It is anotherobject of this invention to provide a coring arrangement for use incasting turbine blades having a plurality of passages therein in aninvestment molding process, whereby the positions of the passages areaccurately controlled and which may be conveniently and efiicientlyhandled in investment molding procedures.

These and other objects are accomplished by providing a core assemblycomprising a plurality of elements including a top and bottom coreblock, each of which is finished to provide locating surfaces forpositioning and supporting the core assembly in an investment mold, aplenum chamber core element, a fluid entry element which extends throughand is cemented within a slot provided in the bottom block and whichextends into and is cemented in a slot in the plenum chamber element,and a plurality of refractory tubes which are cemented in recessesdisposed at the periphery of the plenum chamber element and extend to anarcuate slot in the top core block. The arcuate slot in the top core andthe recesses of the plenum core element are arranged in a manner suchthat the tubular core elements, when fitted in place, extend through theairfoil from the periphery of the plenum chamber to the tip of the bladeto be made, while being suitably spaced from the walls of the airfoil toprovide the airfoil with sufficient strength and a substantially uniformrate of thermal conductivity.

In the process of casting a blade using the above-described coreassembly, a wax pattern is molded about the portion of the assemblybetween the top and bottom blocks which define the external shape of theblade. After the wax pattern is formed, suitable gating portions areattached to the pattern and the wax pattern is invested in aconventionalmanner. of the pattern is removed, the tube passage elements, the plenumchamber element and the fluid entry element of the core assembly arefirmly supported within the investment mold cavity by means of the topand bottom core blocks. After the casting procedure the core elementsare removed by suitable means depending on the type of core materialsused.

These'and other objects and advantages of the invention will more fullyappear from the following detailed description of the invention,reference being made to the accompanying drawings in which:

FIGURE 1 shows an exploded view of the core assembly involved in thepresent invention.

FIGURE 2 shows a perspective view of the core elements in assembledrelation.

FIGURE 3 shows a core assembly having a wax pattern formed therearoundwhich in turn is invested in a refractory mold.

FIGURE 4 is a hollow turbine blade formed in accordance with theinvention.

FIGURE 5 is a top view of the blade taken along the line 55 showingtubular passages leading to the tip of the blade.

In general the present invention is concerned with a method of castinghollow turbine blades, compressor blades or the like by investmentmolding procedures. As shown in FIGURES 4 and 5, the blade of thepresent invention involves an airfoil it a shoulder portion 14 and aroot portion 12 having suitable serrated or ridged flanks adapted tosecure the blade in a correspondingly formed groove or slot in a rotordrum or wheel. The blade is provided with coolant passages 16 extendingfrom the tip 18 of the blade to a plenum chamber 113 located within theshoulder 14 of the blade. A fluid entry opening 15 is provided to theplenum chamber 13 through the root 112. The opening may extend throughthe root longitudinally of the blade as shown, or alternately it maytake the form of a transverse opening through the root portion leadingto the plenum chamber.

An important feature of the invention resides in a core assembly, thecomponents of which are shown in FIGURE 1. The core assembly consists ofa top block 20 of a generally fiat configuration of substantialthickness and having an arcuate opening 22 formed therein, a pluralityof lineal tubular core elements 24, a plenum chamber element 26 of asubstantially fiat and irregular configuration having a slot 23therethrough and a plurality of recesses 30 arranged thereabout, a flat,elongated fluid entry element 32 and a bottom core block 34 having anelongated slot 36 therethrough.

The top block 20 is preferably formed in a generally rectangular shapeand of substantial thickness which may readily be molded and firmlysupported within an investment mold. The arcuate slot 22 is of a shapesuch as to support the tubular passage elements 24 in a positioncorresponding to the desired position of the openings 16 at the tip ofthe blade 18 as shown in FIGURE 5; These openings are preferably locatedmidway between the sides of the airfoil at the tip thereof. The arcuateslot 28, as shown in FIGURE 1, is preferably provided with a somewhat Vor hourglass configuration so as to permit the fluid passage elements 24to be readily accurately located therein.

The recesses 38 of the plenum chamber element 26 are located about theperiphery thereof and are adapted to support the lineal passage formingelements 24 in a After the wax portion manner such that the passagesformed thereby, although lineal, will pass through the curved blade andfollow the contour thereof and extend from the plenum chamber to the tipthereof. It will be noted that the above-described character of the topblock opening 22 and the plenum chamber core element 26 makes itpossible to vary widely the number and character of the fluid passagesto be formed in the blade while utilizing the same basic core structure.The slot 23 of the plenum chamber element and the elongated slot 36 ofthe bottom block are formed to snugly receive the fluid entry element32.

The core elements are preferably made of a suitable material which willwithstand the molten casting metal, and which may be readily removed bysuitable solvents such as caustic solutions which will not appreciablyattack the cast metal. Various compounds of silicon ineluding quartz,borosilicate glass and other glasses or suitable ceramic materials maybe used. Any suitable extrusion casting or injection molding method maybe employed in the manufacture of the core elements. Corning GlassCompany Vicor glass tubing is satisfactory for this purpose. Ceramicmaterials composed of aluminum, silicon and manganese, and sold by theAmerican Lava Company as Al Si Mag 145 and Al Si Mag 670 are suitablefor this purpose. These materials may be disintegrated by leaching witha caustic solution.

In the process of casting a turbine blade in accordance with the presentinvention the core elements are assembled as shown in FIGURE 2 and arecemented together. A refractory cement such as an air setting sodiumsilicate cement is suitable for this purpose. The fluid entry element 32preferably is of sufiicient length to extend through the bottom block 34and the plenum chamber element 26 and to extend substantially beyondthese elements to provide the structure with rigidity and to facilitatethe assembly of the elements. The fiuid passage elements 24 arepreferably tubular and the fluid entry element 32 is preferably providedwith passages 33 therethrough to permit the chemical solvent to enterthe elements to facilitate the removal of these elements from thecasting by a leaching process as will be more fully describedhereinafter.

After the core is assembled, a destructible pattern 46, as shown inFiGURE 3, is formed about the core assembly between the top and bottomcore blocks, and the pattern is invested in a refractory mold 38contained within a metallic container 40 including a base plate 43. Thecore assembly illustrated, involves the fluid entry core 32 whichprovides an opening in the root of the blade longitudinally of theairfoil portion. However, it is obvious that the fluid entry core 32 maybe readily modified to provide a fluid entry passage transversely of theroot. A gating portion 42 having a pouring basin 44 at its outer end isnext attached to the pattern, the portions 42 and 44 being formed of adestructible material similar to that of the pattern. It will beobserved that the destructible portion of the pattern is cast betweenthe top and bottom core elements 2%) and 34, respectively, and thesecore elements are firmly embedded and supported in the refractorymaterial 38 of the mold.

The pattern 46 is preferably formed of a low fusing substance such aswax or a thermoplastic material, or any other vaporizable, fusible,combustible or otherwise destructible material. However, wax or plasticpatterns are preferably employed in order to obtain optimum results.Among the plastic patterns which are satisfactory are those formed ofpolystyrene, although other thermoplastic pattern materials such asresinous, polymerized derivatives of acrylic acid and methacrylic acidmay be used.

The surfaces of the pattern are next coated with a ceramic wash orcoating material which is to provide the smooth casting surface on therefractory mold to be formed. This coating material comprises an aqueousdispersion of conventional finely comminuted refractory materials, abinder, such as an air-setting silicate cement, and defoaming andwetting agents.

Coating of the pattern with the ceramic wash is preferably accomplishedby dipping the pattern in the coating solution. Although in someinstances the ceramic coating may also be applied by spraying orpainting it on the pattern or in any other suitable manner, dipping ispreferred because it assures more uniform coating of all the patternsurfaces and is the simplest method of application.

The dip coat slurry is preferably kept in constant motion by stirringmeans except during the actual dipping operation. However, the mixingaction should not be such as to unnecessarily introduce air into theslurry, and care should be exercised in immersing the pattern in theslurry to prevent air entrapment on the pattern. Normally the dip coatsolution is retained at room temperature during the dipping operationbecause excessive heat can result in distortion of the plastic or waxpattern. The excess coating material is permitted to drain off prior tosubsequent treatment and investment.

After the pattern has been completely coated with the dip coat slurry,it may be sanded or stuccoed to provide a rough surface on the coating,thus insuring greater adhesion between the principal refractory portionof the mold and the dip coat on the pattern. This sanding" may beaccomplished by merely screening or otherwise applying silica sand orother suitable refractory materials in known manner to the outer coatedsurface of the destructible pattern. When all the molding surfaces ofthe pattern have been effectively covered with sand, the pattern andembedded core should be air dried.

Following the formation of the pattern as above-indicated, a suitablemold 3S usually containing a relatively coarse refractory material isformed about the pattern 46 and the gating portion 42 thereof, thelatter being permitted to extend through the wall of the refractory moldso as to permit the escape of the destructible pattern material and toform an ingate for the fluid casting metal. This main refractory moldmay be formed about the pattern in any suitable manner and hence, theprocedure for forming the mold will not be described in detail. Amongthe procedures for forming the body of the mold 38, however, is that ofmixing the refractory mixture with a predetermined quantity of a liquidbinder, pouring it into the sleeve or flask 40 which is preferablyvibrated during this pouring operation and then allowing the mold toset. The mold body 38 may be formed of a conventional silica having anethyl silicate binder or may be formed of any other suitable investmentmaterial. An example of an investment dry mix or grog which may be usedis one comprising major proportions of a finely ground, dead burned fireclay and silica flour and minor proportions of magnesium oxide and boraxglass. The binder for this grog may include an aqueous solution ofcondensed ethyl silicate, ethyl alcohol and hydrochloric acid.

When the mold body has solidified or set to a sufficient extent, thebase plate 43 is removed from beneath the mold and heat is applied tomelt the pattern. It is necessary to apply suflicient heat to raise themold temperature above the fusion point of the pattern material, thuspermitting the molten material of the pattern to escape through the gatein the mold formed by the pattern portions 42 and 44.

Upon removal of the pattern from the mold in the foregoing manner, themolten casting metal is poured or otherwise introduced into the moldcavity formed by the pattern. In the majority of instances, it isnecessary to pour the casting metal while the mold is still hot. Afterthe molten metal has been poured and the casting solidified, therefractory mold body 33 is broken away to permit removal of the casting.The top and bottom core blocks 20 and 34 are also broken away from thecasting in the shake out process.

The casting containing the remaining core elements therein is thenimmersed in a suitable solvent solution whereby the core elementscontained in the body of the casting are dissolved or leached out toform the hollow cast blade. The choice of leaching solution will, ofcourse, depend on the particular core material used.

While the present invention has been described by means of certainspecific examples, it will be understood that the scope of the inventionis not to be limited thereby except as defined in the following claims.

I claim:

1. A core assembly for making a hollow turbine blade including a bladeportion and a root portion, a plenum chamber within the upper portionsof said root portion, a fluid entry passage through said root portion tosaid plenum chamber and a plurality of coolant passages eX- tending fromsaid plenum chamber to the tip of the blade comprising top and bottomcore blocks and a plenum chamber core disposed therebetween in spacedrelation, a plurality of rod-like passage cores, said rodlike passagecores at one of the ends thereof being disposed in an arcuate slotWithin said top block and the opposite ends of said rod-like passagecores being supported within peripheral recesses of said plenum chambercore, and a fluid entry core extending within an opening of said plenumchamber core and an opening in said bottom core block.

2. A pattern for use in molding a hollow turbine blade including a bladeportion and a root portion, a plenum chamber within the upper portionsof said root portion, a fluid entry passage through said root portion tosaid plenum chamber and a plurality of coolant passages extending fromsaid plenum chamber to the tip of the blade, comprising a core assemblyincluding top and bottom core blocks and a plenum chamber core disposedtherebetween in spaced relation, a plurality of rod-like passage coressupported Within an arcuate groove of said top block core and peripheralrecesses of said plenum chamber core and a fluid entry core supportedbetween said bottom core block and said plenum chamber core, and adestructible pattern of the blade formed about said assembly betweensaid top and bottom core blocks.

3. A core assembly for making a hollow turbine blade including a bladeportion and a root portion, a plenum chamber within the upper portionsof said root portion, a fluid entry passage through said root portion tosaid plenum chamber, and a plurality of lineal coolant passagesextending from said plenum chamber to the tip of the blade comprising asubstantially flat top core and a substantially fiat bottom core and aplenum chamber core having a substantially fiat configuration disposedtherebetween in spaced relation, a plurality of rod-like passage coreshaving the ends thereof supported within an arcuate slot in said topcore and having the opposite ends thereof supportedwithin recessesdisposed about the periphery of said plenum chamber core and a fluidentry core supported within a slot within said plenum chamber core and aslot within said bottom core, said core elements being firmly cementedtogether.

4. A pattern for use in molding a hollow turbine blade comprising thecore assembly as defined in claim 3 having a destructible pattern of theblade formed about the assembly between the top and bottom core blocks.

References Cited in the file of this patent UNITED STATES PATENTS2,499,977 Scott Mar. 7, 1950 2,679,669 Kempe June 1, 1954 2,687,278Smith Aug. 24, 1954 2,756,475 Hanink July 31, 1956 2,780,435 JacksonFeb. 5, 1957 2,793,412 Lashbrook May 28, 1957 2,817,490 Brofiitt Dec.24, 1957 FOREIGN PATENTS w ea itai J a 1

